Performance of the FAO Penman-Monteith equation under limiting conditions and fourteen reference evapotranspiration models in southern Manitoba

Evapotranspiration is a key component of water and energy balance. An accurate estimate of reference evapotranspiration (ET o ) is important for determining the water demand of field crops, water management, and hydrological modelling. The FAO-PM ET o equation is the standard equation for estimating ET o . Its use is limited by the requirement for too many observed inputs that are not always available in most weather stations. Empirical models requiring readily available inputs have been developed as an alternative. However, their performance is location-specific. Therefore, this study assesses the performance of the FAO-PM ET o computed with limited data and fourteen empirical ET o models. Meteorological data (2012–2019) was analyzed under the semi-arid climate conditions in southern Manitoba. Model performance was assessed using statistical indices, including R 2 , RMSE, NSE, MPE, and MAE. Results showed that ET o estimates under missing wind speed, relative humidity, or solar radiation were acceptable, although model performance decreased with increased missing data yielding average to poor performance. Based on R 2 , RMSE, and NSE values, among the 14 models compared, the best performing models are Valiantzas-1, Valiantzas-3, Irmak, Valiantzas-2, and Priestly-Taylor models. New empirical coefficients were developed, requiring one or two climatic inputs to improve empirical models. Results showed that the calibrated models performed better than the original equation. The sensitivity analysis showed that ET o is most sensitive to maximum temperature ( T max ) and solar radiation ( R s ), followed by vapour pressure deficit (VPD), wind speed ( U 2 ), and minimum temperature ( T min ). Therefore, it is recommended to ensure accurate measurements of temperature and solar radiation for accurate ET o estimates. This study provides alternative ET o models with reasonable and accurate ET o estimates for southern Manitoba and other areas with similar climate characteristics.